The present invention generally relates to a lithium-containing transition metal oxide target optimal for forming a thin film positive electrode for use in a thin film battery such as a three-dimensional battery and a solid state battery, its production method, and a lithium ion thin film secondary battery. In particular, the present invention relates to a target formed from lithium-containing transition metal oxides showing a hexagonal crystalline system, its production method, and a lithium ion thin film secondary battery.
In recent years, there is a rapidly growing demand for a non-aqueous lithium secondary battery as a high energy density battery. This lithium secondary battery is configured from three fundamental components; namely, a positive electrode, a negative electrode, and a separator retaining an electrolyte interposed between these electrodes.
As the positive electrode and negative electrode, a slurry obtained by mixing and dispersing active materials, conductive materials, bonding materials and plasticizing agents (where appropriate) in a dispersion medium is used by being supported by a collector such as a metallic foil or a metallic mesh.
A composite oxide of lithium and transition metal is used as the cathode active material of a battery as represented by cobalt composite oxide, nickel composite oxide, and manganese composite oxide. These lithium composite oxides are generally synthesized by mixing the compound of the main element (carbonate or oxide of Mn, Fe, Co, Ni and the like) and the lithium compound (lithium carbonate and the like) at a prescribed ratio, and subjecting this to heat treatment (oxidation treatment) (refer to Patent Document 1, Patent Document 2 and Patent Document 3).
In addition, proposed is a ternary positive electrode material having a composition of Ni:Mn:Co=1:1:1 in which the Li/metal ratio is 0.97 to 1.03, and capable of obtaining a discharged capacity of 200 mAh/g (refer to Patent Document 4).
Furthermore, proposed are a cathode active material for use in a lithium secondary battery obtained by adjusting the ratio of Mn, Co, Ni to a prescribed ratio and calcinating this in an oxygen atmosphere (refer to Patent Document 5), and a production method of a cathode active material for use in the lithium secondary battery (refer to Patent Document 6).
Under these circumstances, the thinning of electrode films for shortening the diffusion distance of lithium ion is demanded to meet the needs of achieving even higher output of lithium secondary batteries. This is because, if the thinning of electrode films can be achieved, the battery can be miniaturized significantly. In addition, the thinning of electrode films is crucial technology in three-dimensional batteries and solid state batteries.
With the current production methods of electrode films using a cathode active material for use in secondary batteries as shown in Cited Documents 1 to 6, for instance, with a positive electrode, a conductive material (carbon material such as acetylene black or the like) is mixed with the cathode active material, this mixed powder is added to a binder (for instance, fluorinated resin as represented by pVdF) dissolved in an organic solvent (for instance, NMP: N-methylpyrrolidone), evenly kneaded, this slurry is applied on a collector (for instance, Al foil), dried, and subsequently pressed to obtain an electrode film. Thus, the thickness of the electrode will generally be 50 to 100 um, and sufficient thinning of the film cannot be achieved.
As one method of thinning the electrode film, a wet process as represented by the sol-gel method may be considered. Nevertheless, although this wet method is advantageous in that the thin film can be manufactured inexpensively and simply in terms of the apparatus that is used, there is a drawback in that industrial mass production is difficult.
As an alternative method, a method of forming a thin film with a dry method, in particular the use of the sputtering method may be considered. This sputtering method is advantageous in that the adjustment of the deposition conditions is easy, and [films] can be easily deposited on the semiconductor substrate.
Nevertheless, when performing deposition with the sputtering method, a target for supplying elements to be deposited is indispensible. Generally speaking, a target needs to be prepared to match the composition of the film to be prepared, and the target must not cause any problems during the deposition.
Technology of using this sputtering method to deposit a cathode active material for use in a lithium secondary battery is rare. The reason for this is that there is a possibility that a difference will arise in the component composition between the positive electrode substrate for use in a lithium secondary battery of the sputtering target and the deposited cathode active material for use in the lithium secondary battery, and that it is assumed that it is impossible to obtain a high density target of a level capable of achieving uniform deposition. Thus, in a very real sense, it is necessary to overcome these drawbacks.
Some examples of using this sputtering method to deposit a cathode active material for use in a lithium secondary battery are listed below. Nevertheless, all of these examples relate to a limited composition (LiCoO2), and none of these examples disclose a means for overcoming the foregoing problems concerning the sputtering target.
Technology of annealing and crystallizing thin film amorphous of LiCoO2 formed using the sputtering method at 650 to 900° C. in an Ar or O2 atmosphere when preparing a LiCoO2 thin film positive electrode (Non-Patent Document 1), technology of obtaining LiCoO2 thin films with nanocrystalline structure with (104) preferred orientation on Si covered with Pt using RF magnetron sputtering, and reducing the grain size through annealing at 500 to 700° C. (Non-Patent Document 2), technology of biasing the substrate and performing RF sputtering thereto, and thereby forming a LiCoO2 thin film for use in a positive electrode of a microbattery (Non-Patent Document 3), and technology of adjusting the RF output upon forming a LiCoO2 thin film by way of RF sputtering, and obtaining a nanocrystalline thin film with (101) and (104) preferred orientations (Non-Patent Document 4).
The problem arising under these disclosed sputtering methods is the target, and the target greatly influences the deposition characteristics. Nevertheless, the foregoing disclosed documents do not discuss what kind of target is optimal and what kind of production method should be used regarding a target for forming a thin film of a lithium secondary battery positive electrode.    [Patent Document 1] Japanese Patent Laid-Open Publication No. H1-294364    [Patent Document 2] Japanese Patent Laid-Open Publication No. H1-307094    [Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-285572    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2003-59490    [Patent Document 5] Japanese Patent Laid-Open Publication No. H2-221379    [Patent Document 6] Japanese Patent Laid-Open Publication No. 2002-304993    [Non-Patent Document 1] “Characteristics of thin film cathodes according to the annealing ambient for the post-annealing process” Journal of Power Sources 134 (2004)103-109    [Non-Patent Document 2] “Lithium cobalt oxide film prepared by rf sputtering” Journal of Power Sources 128 (2004)263-269    [Non-Patent Document 3] “Bias sputtering and characterization of LiCoO2 thin film cathodes for thin film microbattery” Materials Chemistry and Physics 93 (2005)70-78    [Non-Patent Document 4] “As-deposited LiCoO2 thin film cathodes prepared by rf magnetron sputtering” Electrochimica Acta 51 (2005)268-273